1. Technical Field of the Invention
The present invention relates to cellular telephone networks and, in particular, to the criteria for mobile station reselection of a control or pilot channel in a digital time division multiple access (TDMA) or code division multiple access (CDMA), respectively, cellular telephone system.
2. Description of Related Art
Cellular telephone systems include a plurality of base stations each serving a respective radio coverage area or cell. As a mobile station moves through the cellular service area in idle mode, it receives information identifying neighboring cells and continuously makes signal strength measurements on the control or pilot channels for each neighboring cell as well as the control or pilot channel for its current cell. From these signal strength measurements, and preferably by identifying the control or pilot channel having the strongest signal strength measurement, the mobile station selects a serving base station. This process is well known in the art with respect to a digital control channel and the TIA IS-136 Specification and with respect to the pilot channel in the TIA IS-95 Specification as mobile assisted cell reselection in idle mode.
When an idle mobile station is located on the border between two or more cells, the control or pilot channel signal strength measurements may be nearly the same. This could cause problems with the reselection of a control or pilot channel (and hence a cell) by that mobile station because variations in the measured momentary control or pilot channel signal strength levels when located on or near the border could trigger multiple reselections among and between the control or pilot channels of the two or more neighboring cells (commonly referred to as a "ping-pong" effect). The ping-pong effect is a significant concern because whenever the mobile station engages in a reselection at this location it unnecessarily expends a little more of its stored battery power, and perhaps may be temporarily unavailable for handling a communications task (such a receiving a page or making an access).
The TIA IS-136 Specification attempts to address the ping-pong effect problem by having the mobile station process the control channel signal strength measurements for purposes of control channel reselection in view of a reselection hysteresis. Reselection of a control channel by the idle mobile station occurs when the measured control channel signal strength for the destination cell exceeds that of the currently serving cell by at least the cell re-selection hysteresis. Reselection accordingly does not occur until the mobile station has moved away from the troublesome cell border area and well into the coverage area of the destination cell. A similar procedure is applied with respect to the pilot channel in accordance with the TIA IS-95 Specification.
The cells within a cellular telephone system may be conveniently grouped together for reference purposes by location area or system area (i.e., registration areas), with each such registration area including a plurality of similarly grouped cells. When a mobile station enters into a cell assigned to a different registration area, it must register with the cellular network at the time of control or pilot channel reselection. This registration process consumes a significant amount of both cellular air interface resources (in that a number of communications must occur to process the registration and recognize the mobile station) and cellular system resources (in registration processing and updating the home and visitor location registers). It would be preferred if such resources were not unnecessarily consumed with respect to a mobile station that is located near the boundary of, and perhaps is likely to move often back and forth between two different registration areas. Another problem exists with multiple registrations in that the system may become confused as to mobile station location. This is of primary concern when the cellular network is congested and registration messages are likely to be delayed. Accordingly, there is a need for a cellular control system and method that makes it more difficult for an idle mobile station to engage in control or pilot channel reselection with respect to a cell belonging to a different registration area.